Christian Bale has never exactly been known for his light-hearted persona either on or off-screen, but ironically his turn in Yimou Zhang's sweeping, intermittently brutal war drama might be his most affable in years. He plays John Miller, a mortician who travels to a convent school for girls in the Chinese city of Nanking in 1937, during its violent six-week occupation by Japanese troops.

Screenwriter Liu Heng goes to great lengths to make it clear that Miller's an irresponsible, semi-sozzled buffoon and happy to remain that way, but there's never any doubt as to where the character's ultimately heading. He finds himself feeling increasingly responsible for the girls' safety, and when a gang of Japanese soldiers descend on the convent with less-than-honourable intentions, he's forced to step up and become roughly the sort of hero we expect Christian Bale to be.

It's a ne'er-do-well turned reluctant saviour arc that isn't remotely fresh, but does compel thanks to Bale's heartfelt and determinedly unshowy performance. Between all the various shades of psychotic yuppie he's played over the years, it's been easy to overlook the salt-of-the-earth decency he's capable of conveying, and it's this that makes his interactions with his young charges as moving as they are.

The film's more pressing problems come with the arrival of a group of local prostitutes seeking shelter inside the convent, creating a frisson that's never really explored as deeply as it could be.

There's the predictable love interest strand between Bale and ringleader Yu Mo (Ni), who has the good narrative fortune of being both the most beautiful and the most secretly troubled prostitute of the bunch. But what really might have made for a compelling core is the strand that remains frustratingly under-explored. The friction between the two groups of young women - for want of a better term, the Madonnas and the whores - is established early on but returned to only in the most shallow way possible.

There's a crucial bleed between the two groups and an inevitable revelation that they really aren't so polarised after all, but this is all third act payoff that would have felt more earned if the girls' relationships had ever seemed like much of a point of interest for the filmmakers.

It's not surprising that Zhang, who's best known for helming Hero and House of Flying Daggers, should ultimately reveal himself to be more interested in aesthetics than characters, and there are enough moments of visual splendour here - alongside jolts of abrupt brutality - to sustain The Flowers of War throughout its substantial running time. Nevertheless Bale is the only actor given what feels like a coherent character arc, and setting aside the troubling implications of that, it's tough to invest when a tragic story is painted in strokes this broad.

The Flowers of War is showing exclusively through Empire Cinemas from August 3 and is released on DVD and Blu-ray on August 6.